There is a growing need for sustainable aquaculture systems. If the aquaculture industry is to keep pace with global demand, new production facilities will need to be built. Technology and protocols that will enable the aquaculture industry to expand production of aquatic species in recirculating systems are essential. Although many conventional aquaculture systems are known, optimal growth of aquatic species is related to many environmental concerns such as excessive water consumption and generation of large quantities of waste by-products such as high nitrogen content effluent. Consequently, the aquaculture industry has recently received considerable criticism due to perceived negative environmental effects. As a result, more stringent effluent regulations for wastewater discharge have been initiated. In addition to a growing demand for aquaculture, there is also increasing demand from consumers for products grown in environmentally responsible systems. In order to meet these demands, new and existing production facilities will need to have limited water usage and low nitrogenous waste effluent as critical components of environmentally responsible sustainable aquaculture systems.
High demand for sustainable aquaculture has resulted in aquaculture farmers using high protein feeds in large quantities to increase fish growth rates and thus, increase profitability. The use of high protein feeds, however, leads to the generation of significantly more nitrogenous waste (i.e., ammonia and nitrate) which must be removed from the system or discharged. In a recirculating aquaculture system, removal of nitrogenous products such as ammonium, nitrate, nitrite or other waste by-products is essential in order for aquatic species to thrive. In recirculating aquaculture systems, ammonia is oxidized to nitrite (NO2−) by autotrophic bacteria in an aerobic biofilter. However, the nitrite generated is also toxic, so a second bacterium is required to oxidize the nitrite to nitrate (NO3−). While nitrate is considerably less toxic than either ammonium or nitrite, elevated nitrate concentrations are still a significant concern for a number of commercially relevant aquatic species.
Nitrogen from nitrate, or nitrate-nitrogen, must be removed from the system, either through water exchange or through denitrification processes. Water exchange has traditionally been used to remove nitrate-nitrogen from recirculating aquaculture systems, although this method has several disadvantages, including the need to discharge or dispose of the large volumes of water used in the exchanges, and thus is not responsive to environmental concerns of excessive water consumption, a problem of particular importance in certain areas or seasons in which water usage is restricted. The discharge of large amounts of nitrogenous products and nitrogenous waste generated through aquaculture systems is harmful unless it is first captured and treated. High nitrogen levels generated in aquaculture systems impact the environment and reduce efficiency and effectiveness of aquaculture systems. Thus, attempts have been made to denitrify, or reduce the nitrate-nitrogen concentrations, of the discharge.
Although denitrification is an effective means of reducing the concentration of nitrate-nitrogen with limited water usage, very few aquaculture operations utilize denitrification because the disadvantages often outweigh the advantages. Since denitrification is an anaerobic process, this process is not typically included in aquaculture systems that require oxygenated water. Imbalance in water chemistries is common in aquaculture systems that use denitrification, which can lead to high concentrations of nitrite or hydrogen sulfide, which are highly toxic to fish. In order to facilitate denitrification and reduce the risk of incomplete denitrification, a carbon source, such as alcohols, volatile fatty acids, starches, or sugars, is required. The use of these carbon sources can lead to bacterial blooms, toxic by-products such as ammonia, nitrite, hydrogen sulfide, among other problems, and result in elevated system costs. Regulation of the amount of carbon added is critical to proper removal of nitrate-nitrogen through biological denitrification.
It would be advantageous to develop a system and method that uses denitrification to maximize the benefits of efficient nitrate-nitrogen removal but at the same time, overcomes shortcomings of prior recirculating systems which utilize denitrification. A recirculating aquaculture system of the present invention utilizes denitrification to reduce nitrate (NO3) or nitrate-nitrogen (NO3—N) concentrations and as a result, nitrogenous waste, and demonstrates greater mitigation of harmful water chemistries or compounds, such as toxic levels of hydrogen sulfide, ammonia, and nitrite, which are a common problem in existing recirculating aquaculture systems which utilize denitrification processes. An aquaculture system of the present invention places denitrification reactors for nitrate-nitrogen removal upstream of aerobic nitrification reactors, which is a novel sequence of reactors not seen in existing recirculating aquaculture systems. In existing aquaculture systems, anaerobic denitrification reactors are placed downstream of aerobic nitrification. The novel arrangement of components of the present invention overcomes some of the limitations of prior aquaculture systems utilizing denitrification reactors, these limitations being high water consumption, large quantities of nitrogenous discharge and risk to survival of aquatic species if an imbalance of critical water chemistries and other parameters occurs. An aquaculture system of the present invention utilizes a mechanical means for removing particulate matter, one or more anaerobic denitrification reactors, followed by aerobic nitrification to provide efficient and inexpensive removal of nitrogenous materials and/or mixtures thereof from aqueous medium by converting said materials to nitrogen gas which is vented to the atmosphere.
A system of the present invention has advantages over other existing aquaculture systems which use denitrification in that it demonstrates responsible environmental stewardship to efficiently use water resources and limit nitrogenous discharge, factors which are critical to the sustainability and growth of commercial aquaculture, while at the same time effectively reducing nitrate-nitrogen concentrations and increasing safety of aquatic species reared in recirculating aquaculture systems.